The invention relates to a method and an apparatus for removing substance from the surface of a workpiece according to the preambles of the independent claims. A method for surface machining is known from the DE 42 09 933.
A manufacturing method suitable for complex shapes is the removal of substance from the full in accordance with the desired shape. In this case either the generated cavity may be the finally desired end product (for example in the case of a mould) or the remaining substance can correspond to a part to be produced. One type of substance removal particularly for fine structures is the one carried out by means of a laser beam.
FIG. 1 schematically shows a corresponding apparatus. A column 16 is provided with a fixed or shiftable workpiece table 14 on which a workpiece 11 is disposed. A laser head 13 emitting a laser beam 12 for generating a cavity or hollow 10 in the workpiece 11 is also fixed to the column 16. The substance removal is respectively effected from the currently accessible face/surface of the workpiece. The comparably highly energetic laser beam irradiates the currently accessible workpiece surface and leads to a vaporisation or melting of the workpiece substance there. The removal of substance is generally carried out layer-wise. The laser beam is guided across the workpiece surface in a scanning manner by a laser beam guidance, for example so that it scans the workpiece surface in a meandering way and removes a layer by melting and vaporisation in accordance with the desired limits or boundaries at that occasion.
In FIG. 1, the x- and z-coordinates are schematically indicated. The y-coordinate protrudes downwards through the plane of the drawing. The layers to be removed are respectively positioned substantially in the x-y-plane on different positions on the z-axis.
For constructional reasons the working area of the apparatus is generally limited. For example, the laser beam can not be guided or offset unlimitedly far. This is schematically shown in the lower part of FIG. 1. The numeral 13 indicates the laser light source. The numerals 12a and 12b respectively denote the farthest possible offsets of the laser beam in the x-direction (for example by means of mirrors). It may then maximally scan an area Bx in the x-direction. Corresponding considerations also apply to the y-direction. The apparatus thus has a work area B which is generally rectangularly limited and has the dimensions of Bx in the x-direction and of By in the y-direction.
When substance is to be removed in an area of the workpiece 11 which is larger than the work area Bx, By, it is principally possible to change a relative position of the workpiece 11 and the laser head 13 in a working area after the removal of one layer. Thereby the position of the working area on the workpiece is suitably shifted so that now another area on the workpiece can be worked.
This is schematically shown in FIG. 2 is a plan view and a cross sectional view of a cavity 10 to be produced. A flat bowl in which four poles are to remain will be taken as an example. The negative shape resembles a large button. The numerals 21 to 24 denote four positions for the working area of the apparatus. They are selected so that they are directly adjacent to each other and cover the desired area when combined. Each of the working areas has the dimensions Bx and By. This results in a first criterion necessitating a change of the relative position of the workpiece 11 with respect to the laser head 13. It corresponds to the necessity of a shift of the working area relative to the workpiece so that a workpiece surface can be worked which is too large to be completely covered by the working area of the apparatus.
Another criterion for changing the relative position is explained with reference to the cross sectional view of FIG. 2. The numeral 20 denotes the contour of the cavity shown in a plan view above. However, if a smaller cavity is presumed, as, for example, indicated by the broken line 20xe2x80x2, such a workpiece section would be well covered by the work area Bx, By of the apparatus. Nevertheless problems may arise due to the fact that certain sections may be shadowed for the laser light. In the cross sectional view of FIG. 2 this is the case in the sections 26a when the light source is assumed to be disposed centrally above the cavity. Other sections 26b may possibly receive the laser light under so glancing an intersection that defined removal ratios can no longer be set. Therefore it may be desirable to change the relative position even in the case of xe2x80x9csmallxe2x80x9d cavities to avoid shadowing and/or to obtain appropriate impinging or incidence angles of the laser light on the walls of the cavity. Returning to the lower part of FIG. 1 again, this may be otherwise expressed in that the change of the relative position is desirable because certain working positions of the workpiece are desirable in certain partial areas of the working area Bx, By. For example, the left walls of the poles and the angles formed with them on the bottom of the cavity can be accessed comparably well with the right section of the cone shown in the lower part of FIG. 1 while the right walls of the poles to be formed and the angles formed with them on the bottom of the cavity can be advantageously reached with the left section of the cone shown in the lower part of FIG. 1.
The two criteria may, for example, become necessary in combination with each other in the production of the cavity shown in FIG. 2 which is, on the one hand, larger than the working area and, on the other, has inaccessible areas and relatively vertical walls.
When now in the layer-wise removal the relative positions are shifted according to the requirements of the above mentioned criteria, problems may arise due to the fact that certain working sites of the workpiece are always disposed in the same section of the working area of the apparatus. It may now be generally assumed that the working properties and particularly the accuracy are not homogenous for the complete working area Bx, By. Particularly the edge sections are critical in this case as discontinuities of some kind will occur here, be it because the laser beam is switched on or off or because the movement is stopped, for example in the x-direction, and a movement in the y-direction is started. These discontinuities result in production inaccuracies which may disadvantageously add up during the layer-wise creation of the cavity.
It is the object of the invention to provide a method and an apparatus for removing substance from a surface of a workpiece in which manufacturing inaccuracies due to non-uniform removing properties in the working area can be avoided.
Said object is solved by the features of the independent claims. The dependent claims are directed to preferred embodiments of the invention.
According to the invention the relative positioning of the workpiece and the working head is, in addition to one or both the above mentioned criteria, set so that the limits of the working areas come to be positioned on different sections of the workpiece surface for identical working sites (with respect to the x- and y-coordinates).
According to the invention it is taken care that in a lower layer the boundaries of the working area are arranged differently on the workpiece than in previous layer. Possible discontinuities within the working area can not be prevented in this way. However, the discontinuities will not add up but will, on the one hand, be averaged out due to a shift of the boundaries and may, on the other hand, be restrained by control techniques.
It is to be explicitly noted here that at a date very close to the date of filing the present application the applicant filed two other applications relating to methods and apparatuses for laser machining of a workpiece, i.e. the application nos. . . . (xe2x80x9cTiefenmessung u.a.xe2x80x9d (Depth Measurement and Others)) and . . . (xe2x80x9cTrennmittelxe2x80x9d (Separating means)). Reference is herewith taken to the full content of said applications.